1. Field of the Invention
The present invention relates to NOx reduction, more particularly to reduction of NOx by selective catalytic reduction technology including plasma-assisted catalytic reduction (PACR) technology, and more particularly to systems for chemically reducing NOx to N2 and other benign gases in oxygen-rich environments.
2. Description of Related Art
The control of NOx emissions from vehicles is a worldwide environmental problem. Gasoline engine vehicles can use newly developed three-way catalysts to control such emissions, because their exhaust gases lack oxygen. But so-called xe2x80x9clean-burnxe2x80x9d gas engines, and diesel engines too, have so much oxygen in their exhausts that conventional catalytic systems are effectively disabled. Lean-burn, high air-to-fuel ratio, engines are certain to become more important in meeting the mandated fuel economy requirements of next-generation vehicles. Fuel economy is improved since operating an engine stoichiometrically lean improves the combustion efficiency and power output. But excessive oxygen in lean-burn engine exhausts can inhibit NOx removal in conventional three-way catalytic converters. An effective and durable catalyst for controlling NOx emissions under net oxidizing conditions is also critical for diesel engines.
Catalysts that promote the reduction of NOx under oxygen-rich conditions are generally known as lean-NOx catalysts. Difficulty has been encountered in finding lean-NOx catalysts that have the activity, durability, and temperature window required to effectively remove NOx from the exhaust of lean-burn engines. Prior art lean-NOx catalysts are hydrothermally unstable. A noticeable loss of activity occurs after relatively little use, and even such catalysts only operate over very limited temperature ranges.
Such catalysts that can effectively reduce NOx to N2 in oxygen-rich environments have been the subject of considerable research. (For instance, see, U.S. Pat. No. 5,208,205, issued May 4, 1993, to Subramanian, et al.) One alternative is to use catalysts that selectively reduce NOx in the presence of a reductant, e.g., selective catalytic reduction (SCR) using ammonia as a reductant.
However, another viable alternative that involves using co-existing hydrocarbons in the exhaust of mobile lean-burn gasoline or diesel engines as a reductant is a more practical, cost-effective, and environmentally sound approach. The search for effective and durable SCR catalysts that work with hydrocarbon reductants in oxygen-rich environments is a high-priority issue in emissions control and the subject of intense investigations by automobile and catalyst companies, and universities, throughout the world.
In the presence of hydrocarbons, catalysts that selectively promote the reduction of NOx under oxygen-rich conditions are known as lean-NOx catalysts, and more specificallyxe2x80x94SCR lean-NOx catalysts. Selective catalytic reduction is based on the reaction of NO with hydrocarbon species activated on the catalyst surface and the subsequent reduction of NOx to N2. More than fifty such SCR catalysts are conventionally known to exist. These include a wide assortment of catalysts, some containing base metals or precious metals that provide high activity. Unfortunately, just solving the problem of catalyst activity in an oxygen-rich environment is not enough for practical applications. Like most heterogeneous catalytic processes, the SCR process is susceptible to chemical and/or thermal deactivation. Many lean-NOx catalysts are too susceptible to high temperatures, water vapor and sulfur poisoning (from SOx). Catalyst deactivation is accelerated by the presence of water vapor in the stream and water vapor suppresses the NO reduction activity even at lower temperatures. Also, sulfate formation at active catalyst sites and on catalyst support materials causes deactivation. Practical lean-NOx catalysts must overcome these problems simultaneously before they can be considered for commercial use.
Some hydrocarbons may be better reductants or better NOx to N2 promoters. Many lean-NOx catalysts have been tested with propylene as the reductant. A disadvantage of such an embodiment is that two different supplies of hydrocarbons must be maintained aboard a diesel-powered vehicle. The preferred embodiment is the use of fuels, such as No. 1 or 2 diesel fuels, as reductants with the lean-NOx catalyst to reduce NOx and concurrently provide fuel for the upstream exhaust-generating engine. Thus, only one uncombusted source of hydrocarbons needs to be maintained aboard the vehicle. Most of the lean-NOx catalysts that have been shown to be efficient with propylene as reductant are not efficient when used with the heavy hydrocarbons present in diesel fuel. There is a great need to find a lean-NOx catalyst that can reduce NOx efficiently using heavy hydrocarbons similar to those present in diesel fuel.
The U.S. Federal Test Procedure for cold starting gasoline fueled vehicles presents a big challenge for lean-NOx catalysts due to the low-temperature operation involved. Diesel passenger car applications are similarly challenged by the driving cycle that simulates slow-moving traffic. Both tests require reductions of CO, hydrocarbons, and NOx at temperatures at or below 200xc2x0 C. when located in the under-floor position. Modifications of existing catalyst oxidation technology are successfully being used to address the problem of CO and hydrocarbon emissions, but a need still exists for improved NOx removal.
The present invention provides a method for reducing NOx emissions and a vehicle with reduced NOx emissions. The present invention also provides a system for attachment to an engine with an oxygen rich exhaust for the reduction of NOx emissions.
Briefly, in a lean NOx selective catalytic reduction system of the present invention, NOx (usually in the form of NO and preferably NO2) is reacted on a high-surface-area, large-pore, basic catalyst, such as an alkali metal-exchanged X-zeolite or Y-zeolite, and converted to environmentally benign products. The invention preferably comprises a non-thermal plasma gas treatment of exhaust NO to produce NO2 which is then combined with the selective catalytic reduction treatment, e.g., a SCR lean NOx catalyst, to enhance NOx reduction in oxygen-rich vehicle engine exhausts. An engine controller can continually or periodically run brief fuel-rich conditions that provide hydrocarbon reductants for a reaction that catalyzes the NO2 (produced by a plasma) into benign products such as N2. By using a plasma, the SCR lean NOx catalyst may contain less or essentially no precious metals, such as Pt, Pd and Rh, for reduction of the NO2 to N2.
Accordingly, an advantage of the present invention is that a method for NOx emission reduction is provided that is inexpensive and efficient. The plasma-assisted lean-NOx/basic zeolite catalyst system can not only remove the dependence on precious metal lean-NOx catalysts, but allows for relatively more efficient compliance with NOx emission reduction laws.
Furthermore, not only does the plasma-assisted lean NOx/basic zeolite catalyzed process improve the activity, durability, and temperature window of SCR/lean-NOx catalysis, but it also allows the combustion of fuels containing relatively high sulfur contents with a concomitant reduction of NOx, particularly in an oxygen-rich vehicular environment. The present invention allows the use of a lean NOx catalyst to reduce NOx emissions in engine exhausts containing relatively high concentrations of sulfur, such as greater than 20 ppmw sulfur (calculated as S).
Still another advantage of the present invention is that an efficient method for NOx emissions reduction at relatively low temperatures is provided using heavy hydrocarbons as the reductant.